1. Field of the Invention
The present general inventive concept relates to a fixing unit used with an image forming apparatus and an image forming apparatus having the same and using an electrophotographic method to point an image on a sheet of paper.
2. Description of the Related Art
An electrophotographic image forming apparatus is a device that prints a single or multiple color image on a sheet of paper (or other image recording media) by forming an electrostatic latent image by radiating light onto a photosensitive medium charged to a predetermined potential, developing the latent image into a toner image using toner of a predetermined color, and transferring and fixing the toner image to the sheet of paper. The printed sheet of paper is discharged along a discharge path.
FIG. 1 is a sectional view of a discharge path of a conventional image forming apparatus, and FIG. 2 is a plan view of FIG. 1.
Referring to FIG. 1, there are shown a heat roller 71 and a pressure roller 72 for applying heat and pressure on a toner image attached onto a sheet of paper S by electrostatic attraction in order to fix the toner image on the sheet of paper S. Reference numeral 40 is a guide member for leading the printed sheet of paper S to the discharge path 20. Reference numeral 50 is a discharge roller for releasing the sheet of paper S. The guide member 40 is fixed at an outlet of the heat roller 71 and the pressure roller 72. As shown in FIG. 2, a plurality of guide members 40 are installed in a width direction of the sheet of paper S. The sheet of paper S passing between the heat roller 71 and the pressure roller 72 contacts a first plane 41. Since the guide members 40 are fixed, the sheet of paper S becomes bent along the first plane 41 and is led to the discharge path 20. Due to this structure, the first plane 41 repeatedly contacts the sheet of paper S so that paper dust or toner residues are separated from the paper S and adhere to the paper S.
The procedure by which toner adheres to the first plane 41 will be described below. The sheet of paper S can be a new sheet or a used sheet of paper of which an image has been already printed on a surface. If the used sheet of paper is heated or pressed between the heat roller 71 and the pressure roller 72, the toner image formed on the surface of the paper is melted and softened. In this state, when the sheet of paper S reaches the first plane 41, its back rubs against the first plane 41 so that the toner adheres to the first plane 41. Such a phenomenon where toner adheres to the first plane 41 becomes severe in duplex printing. This is because the sheet of paper S where an image is printed on one side enters a printer unit (not shown) through a returning path 30, and then a toner image is transferred thereto and heated once more by the heat roller 71 and the pressure roller 72 before the heat previously supplied by the heat roller 71 and the pressure roller 72 is cooled.
When more toner or paper dust adheres to the first plane 41, it is more likely that paper becomes jammed. For example, a front end of the sheet of paper S cannot smoothly pass through the paper path when toner is irregularly attached to the first plane 41. As shown in FIG. 1, the guide members 40 are installed near the heat roller 71 and the pressure roller 72 so that they have a temperature higher than other portions of the image forming apparatus due to the heat of the heat roller 71 and the pressure roller 72. Toner, which is usually in the form of a resin of a predetermined color, becomes sticky when not completely cooled. Therefore, a paper jam may occur because the friction between the sheet of paper S and the first plane 41 to which the toner has adhered increases.
Such a paper jam may occur in the discharge path 20. The toner adhering to the first plane 41 becomes soft by the heat of the fixer 70 and is transferred to the front end of the sheet of paper S when there is a friction between the front end of the sheet of paper S and the first plane 41. The traveling path of the front end of the sheet of paper S is changed as the front end of the sheet of paper S contacts an upper portion B of the discharge path 20 in order to change its course. At this time, toner may adhere to the upper portion B of the discharge path 20. If more toner adheres to the upper portion B of the discharge path 20, the front end of the sheet of paper S is caught in the upper portion B of the discharge path 20, thereby causing a paper jam.